Aqueous dispersions of cationic polymers are, for example, used as retention aids in paper manufacturing industry. Other uses are, for example, as flocculants for treating wastewater, as thickeners, and soil improving agents. Generally, these polymer dispersions comprise a dispersed polymer and a dispersant in which the dispersant usually is a polymeric dispersant. These polymer dispersions can be prepared by polymerising a reaction mixture of water-soluble monomers in the presence of a salt. Finished polymer will precipitate from the aqueous salt solution and, by using a suitable dispersant, form a polymer dispersion.
There are a number of criteria that the polymer dispersion should fulfil to give good results in the final application and be of commercial interest. Such criteria are, for example, the process viscosity, active content, stability, good retention properties, and easiness of preparing the polymer dispersion including preparing the dispersant. Also, criteria such as environmental and safety aspects are of importance.
By process viscosity is meant the viscosity of the reaction mixture when producing the polymer dispersion. The viscosity should be kept low and viscosity peaks should be avoided, or at least reduced as much as possible, during the production of the polymer dispersion. EP 0630909 B1, discloses a process for preparing a dispersion of a water-soluble polymer comprising polymerising a water-soluble monomer in an aqueous reaction mixture containing a salt.
The shelf life of the dispersion, i.e., the stability of the polymer dispersion over time, is an important property. An efficient dispersant is needed for keeping the polymer particles stable in dispersion without settling as sediment. U.S. Pat. No. 6,221,957 discloses an aqueous saline polymer dispersion where the dispersant is based on a cationic polymer containing hydrophobic units, According to the document, the reason for imparting hydrophobic units is to increase the viscosity of the dispersant polymer, which is said to improve the stability of the dispersion. However, a too high viscosity of the polymer dispersion is not beneficial to the end-application. Also, the inclusion of hydrophobic units in the dispersant polymer requires the dispersant to be produced in organic solvents such as ketones, alcohols and ethers. These solvents have to be removed before using the dispersant in aqueous polymer dispersions, which requires additional processing steps. The organic solvents have also environmental drawbacks and can be inflammable which is negative from a safety point of view.
A further important factor to consider Is the active content i.e., the amount of dispersed polymer in the polymer dispersion. A higher active content gives lower transportation costs and easier handling at the end-application. By using an efficient dispersant, dispersions with a higher active content can be obtained at the same time the viscosity can be kept low. However, it may be difficult to combine a high active content with good performance in retention and dewatering in a papermaking process.
The cationic charge of a dispersed polymer effects its ability to form stable dispersions. There exist reasons for providing stable polymer dispersions with a comparatively low cationic charge. Such reasons are, for example, FDA limits of cationic monomers for certain use, cost, risks of overcharging the cellulosic suspension when used in papermaking.
During preparation of a polymer dispersion, deposits of polymer may form and stick to the reaction vessel and stirrer. This leads to time consuming cleaning procedures of the reaction equipment.
It is an object of the present invention to provide a process for preparing a polymer dispersion in which the process viscosity is kept low and smooth during preparation without any large viscosity peaks, and which gives no deposits. There is a further object of the present invention to provide a polymer dispersion having high stability, high active content with comparatively low cationic charge, and which at the same time gives good retention when used in papermaking processes.